A series of 1,4,7-triazacyclononane based ligands containing one, two, or three methylphosphinate (MP) side-chains has been prepared. The macrocycle with three methylphosphinates, NOTMP, appeared to be a very promising ligand for monitoring MgII in biological samples by 31P NMR spectroscopy. The 31P resonances of the free ligand and the MgII complex were well resolved, in slow exchange, and well downfield of typical tissue phosphate and phosphate ester metabolite resonances. The Kd value of the Mg(NOTMP) complex was 0.35 mM at physiological pH and temperature, a value which is optimal for accurate assessment of free MgII in cells and blood plasma. The latter was confirmed experimentally. Furthermore, the selectivity of NOTMP for MgII over CaII exceeded two orders of magnitude. The Kd values of the MgII complexes were sensitive to pH and temperature. The pH effects could be easily predicted from potentiometric pH titration data. NOTMP is now being used to measure free, uncomplexed MgII in plasma of patients from the cardiac care unit. We have also continued to explore the utility of TmDOTP5- as a NMR shift reagent for resolving intra- and extracellular 23Na signals in vivo. The interaction between Li+, Na+, Cs+, Ca2+, Mg2+ and the shift reagent has been studied by 7Li, 6Li, 23Na and 133Cs multinuclear NMR spectroscopy. The unusually large paramagnetic shifts ( >> 100 ppm) observed in the cation NMR resonances at low alkali metal ion to TmDOTP5- ratios indicated that the cations bind near the 4-fold symmetry axis of the complex. The geometric parameters for the adducts formed between 7Li, 6Li, 23Na and 133Cs cations and TmDOTP5- were obtained from shift and relaxation rate data and compared with those calculated using MMX energy minimization techniques. It was found that the larger Cs+ ion lies closest to the 4-fold axis of symmetry and displays the largest binding constant, suggesting a greater contribution from tetradentate coordinated species. The smaller Li+ ion deviates most from the 4-fold symmetry axis and has a slightly less favorable binding interaction than Cs+ due to a greater contribution from tridentate coordinated species. Na+ lies somewhere between these two extremes and likely averages bidentate coordination. A significant reversal of the 23Na shift upon addition of Ca2+ and Mg2+ indicate that these two divalent cations form very stable 1:1 adducts with TmDOTP5-(logK (Ca2+) ~ 4 and logK (Mg2+) ~ 3).